Optical device and method of manufacture

ABSTRACT

The present invention provides for a method of producing an optical device by means of electron beam lithography and including the step of varying the characteristics of the electron beam spot during formation of the device and also an apparatus for producing diffractive optical devices and/or holographic devices by means of electron beam lithography and including an electron beam lithograph, controlling and processing means, means for varying the characteristics of the electron beam spot during formation of the device, and wherein the processing means is arranged for compiling and pre-processing data and for providing optimisation and allocation control and to optical devices such as those produced thereby.

[0001] This invention relates to optical devices and related methods ofmanufacture and verification. In particular, but not exclusively, theinvention relates to security anti-counterfeit devices employing theprinciple of optical diffraction, and to an improved form of opticalsecurity device for use in the protection of documents and articles ofvalue from counterfeit and to verify authenticity.

[0002] Several forms of such devices are known and used to prove theauthenticity of items of value and to prevent their fraudulentduplication. Examples of such items are banknotes, plastic cards, valuedocuments such as fiscal stamps, travel documents such as passports andalso valuable goods per se.

[0003] Devices based on the principle of optical diffraction have founduse for such purposes since they can produce, by the process of opticaldiffraction, an optically variable image with characteristic featuressuch as depth and parallax (holograms) and movement features and imageswitches (purely diffraction grating devices and some holographicdevices). Such diffractive, optically variable image forming devices areused as anti-counterfeit devices both because their effects are highlyrecognisable and cannot be duplicated by print technologies, and becausespecific, and difficult-to-replicate, optical and engineering techniquesare required for their production.

[0004] Such diffractive, optically variable image-forming devices aregenerally manufactured to produce effects based on holographic or purediffraction grating techniques and often comprise embossed surfacerelief structures as those known disclosed in Practical Holography,Graham Saxby, Prentice Hall 1988. These device structures are typicallyapplied to documents of value, plastic cards and articles of value to beprotected in the form of holographic or diffractive hot stamping foil orholographic or diffractive labelling, often tamper evident.

[0005] There are various forms of pure diffraction grating devicesalready known and in use as such security devices. One example isdisclosed in U.S. Pat. No. 4,568,141, which describes a diffractionoptical authenticating element that provides a colour pattern moving ata predetermined velocity along a predetermined track when the documentis illuminated from a first direction, and viewed from a seconddirection. The device described consists of a plane diffraction gratingstructure which defines a predetermined track in which at least one ofthe spatial frequency, or angular orientation, varies along said tracksuch that when the device is illuminated and rotated in a plane,adjacent regions of the diffraction grating structure successivelydiffract light to cause a viewer to see a colour pattern which appearsto move along the movement track. Each element of this device comprisesa pure plane diffraction grating and does not form an image outside ofthe plane of the device.

[0006] A manufacturing method for such a security diffraction gratingmaster is disclosed in U.S. Pat. No. 4,761,252 and employs a techniqueusing a punch to impress successive small areas of a flexible embossingdie into a sheet of thermoplastic material.

[0007] Further, U.S. Pat. No. 5,034,003 discloses another form ofoptical security device using diffraction gratings to produce aswitching image by recording in the device sets of pixels with eachpixel consisting of small areas of different grating spatial frequenciesand orientations and serving to form a diffracted image visible fromdifferent directions. In this device a diffractive device switchesbetween two replayed images created by splitting the device into severalsets of interlocking pixels, with each pixel divided into sub pixels ofplane diffraction gratings with different sub pixels corresponding todifferent replay directions. This device only forms images located onthe image plane of the device. There is no provision for additionalfunctionality within the image such as enhanced view angle or deptheffects to provide parallax effects.

[0008] Another known method of producing a pure diffraction gratingsecurity device is to write directly the diffractive structure by use ofelectron beam lithography, such as those known in the art as Catpix,Pixelgram, and Exelgram. Some examples of this are WO-A-9103747,WO-A-9428444, WO-A-9318419, WO-A-9504948 and WO-A-9502200, WO-A-9823979and WO-A-002067 describing electron beam generated diffractive opticalsecurity devices.

[0009] WO-A-9103737 describes a method of subdividing an opticallyinvariant image into a set of pixels which are mapped to diffractionspatial frequencies where a greyness factor for each pixel determinesthe extent of curvature of the grating lines in each pixel. WO-A-9318419describes a pixellated optical diffraction device in which each pixel isan individual optical diffraction grating where the pixels are arrangedin groups containing a multiplicity of pixels according to apredetermined rule, typically mapping to larger pixels of an opticallyinvariant image, in order to produce a visually observable opticallyvariable effect. WO-A-9428444 describes a pixellated diffractive deviceincluding a multiplicity of pixels in turn divided into multiple subpixel arranged in gridded arrays, where the sub pixels are arranged ingroups to cooperate to generate a multiple image diffractive device uponillumination. It should be noted that all of the above techniques arelimited to subdividing images into rectangular pixels that generallycontain diffraction gratings of various types. WO-A-950498 details thestructure of a surface relief diffractive device which generatesmultiple diffractive image components under illumination where thediffractive structure is divided into multiple discrete tracks.WO-A-9823979 describes the creation of a colour diffractive device byagain splitting an image into a gridded rectangular pixel array andfurther sub-dividing this array into component diffraction gratingswhose relative areas are adjusted to control hue and intensity ofcolour. WO-A-9917941 describes a pixellated diffusing device in whichthe diffuse elements are arranged in pixels and further arranged to havegrey scale regions created by using structures of different scatteringproperties. A disadvantage of the ‘Pixelgram’ image pixel arrangementsknown from the above documents are the discontinuities that are evidentbetween adjacent pixels, and in the Exelgram, the discontinuitiesbetween tracks. These inter-element gaps lead disadvantageously todiffuse scatter effects and extraneous diffraction effects. Additionallysmall pixels or tracks tend to increase scatter and reduce area fillefficiency and brightness. WO-A-002067 describes a diffractive deviceconsisting of background diffractive elements and interstitialdiffractive elements arranged such that the diffraction effect of thebackground elements is modulated by the interstitial elements.

[0010] The teaching of all of the above documents is incorporated hereinby reference.

[0011] Diffractive optical variable image forming devices are also knownand have been produced by holographic methods, such devices are knownfor their use in security applications for example on credit cards,banknotes, etc. Examples of teachings on such holographicallymanufactured security structures can be found in U.S. Pat. No.5,694,229, U.S. Pat. No. 5,483,363 and WO-A-9959036. The opticalrecording and manufacturing methods and other teachings of thesedocuments are incorporated by reference. Again these teachings arelimited particularly in their flexibility and range of covert andmicroscopic features that may be incorporated.

[0012] Some teachings also exist in relation to machine readable, orcoherently viewable, holographic or diffractive structures as used forsecurity. For example U.S. Pat. No. 4,544,266 reveals the authenticationof a document by the use of a machine readable diffraction based encodedmark that is difficult to copy, and U.S. Pat. No. 5,101,184 describesanother way of machine reading a diffractive security device bydetecting the different intensities of diffracted light produced indifferent directions by asymmetric relief structures.

[0013] Another security device consists of a volume hologram andcontains a visually viewable hologram combined with a superimposed lasertransmission hologram as disclosed in DE-A-3840037, where the lasertransmission hologram image is designed to be non-discernable underwhite light but designed to be read under coherent laser light using avisualiser or machine reader device.

[0014] U.S. Pat. No. 5,483,363 describes how an embossed surface reliefhologram can contain a superimposed second diffractive structuredesigned to be machine readable by creating an out-of-plane image.

[0015] Another holographic system and method for determining which of aplurality of visually indistinguishable objects have been marked with acovert indicator is disclosed in U.S. Pat. No. 5,825,475 which reveals anumber of usually indistinguishable objects some of which have beenmarked with a covert holographic indicator which is exposed to be viewedbut which is detectable only when illuminated with a coherent referencelight of predetermined wavelength designed to be evaluated by a specificform of scanner evaluation unit.

[0016] Accordingly, the current invention seeks to provide an opticalstructure and method of producing the same having advantages over knownsuch devices.

[0017] The present invention seeks to provide an optical diffractionstructure having advantages over known such devices and also seeks toprovide for an apparatus and method for producing such opticaldiffraction devices with advantages over other known apparatus andmethods in the field.

[0018] The present invention describes a novel apparatus involving theuse of electron beam lithography for writing diffractive structures inparticular using a writing method where the size and shape of electronbeam spot is entirely flexible and adjusted during the exposure tooptimise the exposure time and parameters as appropriate.

[0019] The current invention also describes a novel data pre-processingmethodology and software routines which enables optimisation of theexposure area to an absolute level of resolution and which enables thecreation of any graphical arrangement of grating or holographicstructures without limitation. This is in contrast to the previousdevices described which were essentially limited to pixel, line or trackstructures.

[0020] Advantageously, the combination of this data pre-processingmethodology and a new flexible e-beam writing technique can be combinedto form a new arrangement and system for the exact direct digitalrecording of any holographic or diffraction grating structure byelectron beam writing of the pattern into, for example, silicon. Such anarrangement allows essentially a nearly unlimited flexibility withregard to the microstructure that can be recorded allowing an enormousrange of optical structures to be recorded and combined together with adiverse range of characteristics (straight lines, continuous curvedlines, geometrical shapes), and both diffraction grating, Fourierhologram and rainbow hologram patterns and optical element patterns canbe recorded into one hologram.

[0021] Furthermore the resolution of graphical motifs achievable inaccordance with the present invention equates to the resolution of themicrostructure of the hologram itself and these patterns can be drawncontinuously and without recourse to previous techniques which involvesplitting graphic designs into line and pixel patterns to recorddifferent effects.

[0022] Within the concept of the present invention there is completeflexibility to create, for example, an image displaying multipleswitching diffractive images at different view angles or apparentmovement or other effects either by creating regions of superposedmicrostructures that replay simultaneously the two or more differentimages or by sub dividing the design into any complex array of graphicalelements (which may themselves contain additional information withintheir spacing, shape and arrangement) and sub-dividing the twodiffractive channels between arrays of these arbitrary sub-elements,ideally according to a predetermined or statistical rule.

[0023] This new technique and the new class of optical devices socreated are particularly suited for high security applications such ascredit cards, banknotes, brand protection and the like because of thewide variation of available effects, the extremely high resolution allowthe creation of very high resolution security features. This enables thecreation of highly distinctive diffractive security images with a veryhigh degree of resistance to counterfeit or reproduction by eitherconventional holographic or diffraction based generation techniques suchas the technique known as dot matrix origination.

[0024] In the known art, data processing techniques for digitally drivenorigination, that is both for dot matrix systems and particularly forelectron beam based systems as noted above, typically rely on softwarebased on two level data input. The first level comprises graphics datasuch as bitmap date, for example; graphics design of hologram ordiffractive structure which is converted to a graphics bitmap with aspecially defined palette. Each pixel of graphic data represents eithera diffraction grating (with variable period and angle of lines) or anyother custom pattern (circular gratings, special graphics, multilevelstructure, etc.). The interior pattern within a pixel or track, usuallya diffractive structure but not limited to this, is defined on thesecond level of data input as shown in FIG. 1.

[0025] Pixel/track based systems have certain limitations from thesoftware/writing approach which make some exposures ineffective, whenconsidering the exposure time, and which cause exposed diffractivestructures to generate additional optical noise.

[0026] Firstly, it should be appreciated that the whole diffractivestructure is being pixelized. Both the graphic pixel structure and writebeam spot size structure cause generation of optical noise. For example,noise will be generated at inter pixel gaps even within continuous areasof gratings because at the boundaries of pixels there is usually someblank area as in FIG. 2 which makes a very fine slit structure over thewhole hologram area and for the microstructure transitions betweenmicrostructure lines between pixels may not be continuous for the phaseconditions reducing the efficiency of the device. This problem occursusually only when the hologram type of structure is exposed, becausediffraction gratings are pre-defined in libraries of gratings with themost effective algorithm which does not fill the whole pixel areacompletely.

[0027] Secondly, the pixel structure of a diffractive device limits thegraphic resolution of the diffractive or holographic image, since eachpixel should contain at least about ten lines creating an elementarygrating. This leads to approximately a 2000-3000 dpi limit in graphicresolution for pixel based devices. For example, the minimum height ofholographic microtext may be 0.1 mm and in this case information aboutfont shape will be lost. The pixel structure can sometimes also limitresolution in spatial spectra of diffraction gratings which fill pixels.In particular it is sometimes not possible to create an arbitrary angleof grating lines (considering pixel size) with optimised diffractionefficiency over a large area due to discontinuities between pixels. Thislimitation is shown in FIG. 3.

[0028] The present invention has advantages over prior techniques toovercome the above-mentioned limitations.

[0029] The invention is described further hereinafter, by way of exampleonly, with reference to the accompanying drawings in which:

[0030]FIG. 4 illustrates an electron beam spot produced according to anembodiment of the present invention;

[0031]FIG. 5 illustrates an area of microstructure;

[0032]FIGS. 6A-6C illustrates electron beam spot parameters as arisingin an embodiment of the present invention;

[0033]FIGS. 7A-7E illustrate various exposure modes in the boarderregions of exposed areas according to embodiments of the presentinvention;

[0034]FIGS. 8A-8E illustrate the filing level of exposure near the saidboarder regions;

[0035]FIGS. 9-11 illustrate the exposure of the structure with atolerance related to electron beam spot size;

[0036]FIG. 12A illustrates an optical microstructure offering twodiffractive images according to an aspect of the present invention;

[0037]FIG. 12B illustrates possible structures for a zone of themicrostructure of FIG. 12A;

[0038]FIGS. 13 and 14 illustrate examples of sub area subdivisionpatterns of the diffractive structure images of FIGS. 12A and 12B;

[0039]FIGS. 15A and 15B illustrate diffractive devices according tofurther embodiments of the aspects of the present invention;

[0040]FIGS. 16A illustrate another example of a device embodying thepresent invention;

[0041]FIGS. 16B and 16C illustrate examples of area subdivisionsemployed within the embodiments of FIG. 16A.

[0042]FIG. 17A illustrate examples of parallax-compensating areas ofsubdivision;

[0043]FIG. 17B illustrates an example of a device employing thesubdivisions of FIG. 17A;

[0044]FIG. 18 illustrates the varying image offered by a deviceaccording to a further embodiment of an aspect of the present invention;

[0045]FIGS. 19A-19C illustrates examples of devices according to furtherembodiments of the present invention;

[0046]FIG. 20 illustrates examples of devices embodying the presentinvention and incorporating microscopic information structures;

[0047]FIGS. 21A-21C illustrates embodiments of structures according tothe authentification aspects of the present invention;

[0048]FIG. 22 illustrates a diffractive structure according to anotherembodiment of the present invention;

[0049]FIG. 23 illustrates a yet further arrangement of subdivisionsaccording to an embodiment of the present invention;

[0050]FIGS. 24A-24C illustrate an optical device embodying furtheraspects of the present invention;

[0051]FIG. 25 is a flow diagram of a process according to one embodimentof the present invention; and

[0052]FIG. 26 is a flow diagram of a process according to anotherembodiment of the present invention.

[0053] Throughout the text, it should be appreciated that the process offorming an embossed hologram or embossed surface relief diffractivestructure is broadly as known in the art (e.g. G. Saxby, ‘PracticalHolography’) after the lithographic exposure step. To form an embossedhologram the surface relief image exposed by the lithograph afterdevelopment is formed in photo-resist—this is would be silvered todeposit a conductive layer, copied probably several times in a platingprocess as known in the field to form metal copies of the structure andthen typically roll embossed into a plastic material or embossinglacquer or hot foil material using a thermo forming process, or castinto an ultra violet curable material and then metallised with typicallyaluminum or another reflective metal, perhaps chromium for example, toform an embossed hologram or embossed surface relief diffractivestructure as known in the art. In an alternative process the surfacerelief diffractive structure can be coated with a transparent reflectorsuch as typically titanium dioxide or zinc sulphide, as know in the art,to form a semi transparent surface relief diffractive structurereplaying under illumination a visual image by the process of opticaldiffraction for example for use as a data protection overlay on adocument. An alternative method of forming a data overlay is to use ademetallisation process to partially demetallise an aluminum reflectorfor example.

[0054] In one aspect of the present invention, e-beam lithography isemployed and in particular the realisation and actualisation of anelectron beam spot size and shape that is continuously variable duringan exposure to optimise the structure and exposure time. A particularlyadvantageous arrangement incorporated into this invention specificallyfor the writing of optical and other microscopic non diffractivefeatures is where the electron beam spot shape is rectangular as shownin FIG. 4 with variable size which can be changed during exposureindependently in both Cartesian coordinates. For example an advantageoussystem detailed here is where the spot size can vary from 0.1 micron to6 micron in both coordinates, the accuracy of the e-beam positioning is0.1 micron, and sharpness in the corners of the spot 0.1 micron.

[0055] As an advantageous feature, the electron beam spot can be rotatedby rotation of the beam about its longitudinal axis.

[0056] In another aspect of the present invention there is provided anew data pre-processing methodology which enables exposure optimisationof arbitrary binary structures specified by bitmaps. So for example byanalysing a microscopic exposure pattern for a diffractive device themethodology can both optimise resolution and exposure time by adjustingelectron beam spot size between spot exposures.

[0057] In one example, an exposure system employing a variable spotshape electron beam and exposure optimization methodology are combinedto create a new and improved method for the direct writing of arbitrarydiffractive or holographic structure specified by bitmaps.

[0058] Such a system can employ the following steps. The optimisationtechnique uses a method of taking coloured areas in the image specifiedby a black and white bitmap and dividing them into divided intoelementary sub-areas, that is into e-beam exposure spots. The divisionof coloured areas in the image is calculated by an optimisation routinemade considering the minimum number of sub-areas (exposure spots)necessary to expose the whole image by use of a variable exposure spotand then by submitting to different optional parameters.

[0059] Step 1. Typical input data for an arbitrary microstructure ablack and white bitmap (“*.bmp” file type), where the white areasrepresent exposure area; or a greyscale bitmap, where the individualgrey levels represent different exposure dosage.

[0060] Step 2. The first step is to define the Input parameters for theoptimisation process and exposure process in terms of:

[0061] minimum and maximum e-beam spot size for optimization as shown inFIG. 6A.

[0062] maximum ratio between sizes in both coordinates as shown in FIG.6B

[0063] Define and limit the allowable change in spot size for example tobe only in multiples of minimum spot size as in FIG. 6C.

[0064] Step 3. The next step is to define the permitted exposure modesnear the border of exposed area in terms of:

[0065] The extent by which exposure spots cannot exceed the border of anexposed area (or only by a defined portion of the minimum exposure areaof 0.1×0.1 micron) as illustrated in FIG. 7A.

[0066] Or an allowable extent by which exposure spots can exceed theborder area, typically by only up to one half of the minimum spot sizeas in FIG. 7B.

[0067] Or exposure spots may exceed the border maximally to the area ofthe minimum spot size as in FIG. 7C.

[0068] Or exposure spots may exceed the border maximally to 0.5 of theminimum spot area as in FIG. 7D

[0069] Or exposure spots may exceed the border maximally to the minimumspot area as in FIG. 7E

[0070] The optimum and preferred edge exposure mode in this invention isthe exposure mode of FIG. 7D where the exposure spots may exceed theborder maximally to 0.5 the minimum spot area. However, it should beappreciated that all modes are possible within the scope of thisinvention.

[0071] Step 4. The next step is to define the filling level of exposurenear the border which can be one of several types as illustrated inFIGS. 8A to 8E.

[0072] maximum filling is illustrated in FIG. 8A.

[0073] incompleteness of filling on the surrounding area of the spot (ineach of four directions) may reach the minimum spot area as in FIGS.8B-8E.

[0074] The optimum and preferred mode of filling level of exposure nearthe border in this invention is the exposure mode A where the exposurearea is maximally filled. However, all modes are possible within thescope of this invention.

[0075] Step 5. The final input parameter is the optimized filling of theexposure area with exposure spot having have an optional exposure dosage(exposure time), which can be assigned by external data.

[0076] The output of the above methodology yields three aspects.Firstly, a bitmap representing the final exposure area. Secondly a setof data file as an input into exposure simulation software and thirdly aset of data files as an input exposure control system of e-beamlithograph.

[0077] Step 6. An optional intermediate step further involves optionallyrunning the set of data files in an exposure simulation softwareprogramme to check for run time and run integrity.

[0078] Step 7. In the final writing step the methodology then involvesusing these data files to write an arbitrary diffractive microstructureusing these data files as the input exposure control to an electron beamlithograph. During the exposure the lithograph and the data files willcontrol exposure time per spot (viz dosage), spot area which will changecontinuously between successive exposures as required to optimise theexposure time, fidelity to the required pattern and fill factors andspot position for each successive stepwise exposure which will determinethe form of the microstructure written.

[0079] After exposure typically into an electron beam sensitivephotoresist the resist will be developed by a process known in the artby an etching process to give a surface relief optical microstructure inthe resist layer. This structure can be metallised and then used in aplating process to forms replica suitable for mass replication by rollembossing, moulding or flat bed embossing.

[0080] The advantage of this technique is that an arbitrarily shaped areof area can be accurately filled with any arbitrary microstructure. Thetechnique allows for complete flexibility but also complete optimisationof the process whilst at the same time retaining an optimised exposuretime because the exposure resolution is retained maximally whererequired and also the electron beam spot size can be increased whereallowable to allow for more rapid exposures. This technique will createany holographic or diffractive structure provided the structure requiredcan be computer calculated and expressed as a bitmap and will alsocreate a wide range of nano-technology structures such micro facets,micro optics, micro-mechanical structures or micro detectors veryaccurately and flexibility.

[0081] In a second aspect of a method embodying the present inventionthe combination of exposure system by variable spot shape electron beamand another exposure optimisation methodology are combined to create anew and improved method for the filling of an arbitrary area with submicrometer structures. This enables the filling of very high resolutionsub areas of a security design with sub micrometer structures taken forexample from a data library.

[0082] This aspect of the invention uses a methodology described bysoftware description for filling exactly a graphic area to the highestpossible resolution obtainable (i.e. to the size of the e-beam spot)with a sub-micrometer structure such as a diffraction grating,holographic structure, diffractive structure or nano-structure whoseform is pre-determined by a data file contained in a data library.Typically the methodology would create a security optical microstructureby combining a graphics file defining the image to be found, adescriptor of each graphical area in terms of a greyscale or RGB scaleand a data library containing various pre-calculated sub-micrometeroptical or holographic microstructures which are assigned to infillspecific area according to a mapping of a greyscale or RGB pallette fromthe master graphical file to the data library. This allows an opticalmicrostructure consisting of many areas of other optical microstructuresto be created from a wide ranging data library of structures. Animportant part of this methodology is the way in which themicrostructures are tailored to permit exact high resolution filling ofan arbitrary area defined by bitmap or vector curve by a customsub-micrometer structure whilst retaining a reasonable write time forthe entire structure. This method overcomes the previous limitations ongraphics resolution and form of graphics (pixels, tracks) used by othersystems by allowing arbitrary graphical regions of high resolutionlimited only by the spot size of the recording system to be exposed.

[0083] The methodology of this aspect comprises the following:

[0084] Step 1: The input graphics are assembled as one of a number ofpossible digital graphical file formats using one of a number ofstandard formats or programmes. Colours in the image represent aspecific area filling as defined either by the user defining a specificarea filling or by a user or job specific algorithm dictated a mappingfunction to the optical microstructure data library for the generationof various specific predetermined elementary optical microstructuremotifs or patterns for filling the area defined by a particular colouror greyscale.

[0085] Step 2: The methodology of filling the structure, the methodologybeing defined such that the structure cannot exceed the border. Theguiding rule methodologies that can be used for this in this system areas follows:

[0086] Exact—the structure is exposed up to the border with an accuracyof the minimum e-beam spot size—typically 0.1 micron—as shown in FIG. 9.

[0087] With tolerance of minimum spot size used as shown in FIG. 10.

[0088] With tolerance of one exposure spot—the structure is exposed upto the border with tolerance of the exposure spot exceeding theborder—as shown in FIG. 11.

[0089] The optimum fill pattern used depending on job requirement wouldbe the first option where the structure is exposed to form graphics ofthe highest possible resolution.

[0090] The output these Steps 1 and 2 comprises a data file used to runan exposure simulation software and also as a data file to the exposurecontrol system of the lithograph. The colours RGB or greyscale of thegraphics file map to specific microstructures in the data file via anassignment which can be operator defined. Known as the exposurepalette—typically up to 5000 assignments are possible with this system.

[0091] Step 6. An optional intermediate step further involves optionallyrunning the set of data files in an exposure simulation softwareprogramme to check for run time and run integrity.

[0092] Step 7. In the final writing step the methodology then involvesusing these data files to write an arbitrary resolution area of a set ofoptical microstructure defined by a data library diffractivemicrostructure using these data files as the input exposure control toan electron beam lithograph. During the exposure the lithograph and thedata files will control exposure time per spot (viz dosage), spot areawhich will change continuously between successive exposures as requiredto optimise the exposure time, fidelity to the required pattern and fillfactors and spot position for each successive stepwise exposure whichwill determine the form of the microstructure written.

[0093] During the exposure the electron beam spot size and shape that iscontinuously variable during an exposure to optimise the structure andexposure time. A particularly advantageous arrangement incorporated intothis invention specifically for the writing of optical and othermicroscopic non diffractive features is where the electron beam spotshape is rectangular as shown in FIG. 4 with variable size which can bechanged during exposure independently in both Cartesian coordinates. Forexample an advantageous system detailed here is where the spot size canvary from 0.1 micron to 6 micron in both coordinates, the accuracy ofthe e-beam positioning is 0.1 micron, and sharpness in the corners ofthe spot 0.1 micron.

[0094] After exposure typically into an electron beam sensitivephoto-resist the resist will be developed by a process known in the artby an etching process to give a surface relief optical micro-structurein the resist layer. This structure can be metallised and then used in aplating process to forms replica suitable for mass replication by rollembossing, moulding or flat bed embossing.

[0095] By means of the above methods, or a combination thereof, anoptical microstructure, or any nano technology scale structure, can beproduced by either using the first method to fill an arbitrary area withan arbitrary sub-micrometer structure, or by using the second method tofill any arbitrary area of a resolution up to the resolution of thee-beam spot with any one off a number of predetermined microstructuresheld in a data library and described by an exposure palette linking thegraphic design file to the data library. The exposure methodology usedthroughout this is electron beam of variable and definable spot size.These methodologies allow the exposure of highest possible resolutionstructure in reasonable times on an electron beam system by tailoringspot size to exposure optimize both resolution of graphics and minimizeexposure time during run). These methodologies allow areas of opticalmicrostructure or nano-structures with the ultimate highest resolutiongraphics to be recorded using electron beam lithography in an efficientway. This overcomes the limitations of previous systems to pixels,tracks or other such field structures.

[0096] The invention also provides for novel diffractive optical devicefeatures that can, in particular, arise from the above-mentioned methodsand systems.

[0097] A diffractive optical security device producing two or moredefined graphical images visible to an observer from differentobservations directions around the device when the device is illuminatedby white light producing a defined image switch and change between twoor more visually distinct graphical two dimensional or three dimensionalviews illustrated in FIG. 12A. This device is characterized in that themicrostructure corresponding to each graphical view in any small area iscontained in a defined discrete small area, characterized that thegraphical areas containing the microstructures are of a flexiblegraphical shape. For example, and as illustrated in FIG. 12B, onediffractive view channel of an image could be in the shape of smalldots, lines, figures or micrographics reversed out of anotherdiffractive viewing channel, characterised such that the size of thestructures would be smaller than the normal resolution of a humanobserver and therefore non visible and non degrading to the graphicalpatterns seen by an observer. A useful and preferred example is wherethe sub division areas corresponding to the various channels are splitbetween defined graphical areas corresponding to different diffractedviews using graphical shapes of various substantially different sizesand shapes generated by a shape generating and an area splitting rule.So these patterns in one preferred embodiment are split not into uniformpixels various and perhaps varying shapes. A useful preferred embodimentof this is where the rule for area division is governed to generate apseudo random pattern whose shape and area is governed by fractalgeometry, where the areas appear random in shape and size but have adiscernable statistical profile. A particularly preferred embodimentwould be where the fractal pattern has been generated according to aparticular rule carrying particular characteristic information which canbe decoded by analyzing the pattern by the use of fractal techniques.One such fractal type encoding is used in the a technique known as‘Microbar’ for the protection of data by the incorporation of hiddendata encoded and encrypted within the fractal pattern but decodableusing the correct algorithm by analysis of the pattern. So the switchingimage becomes no longer pixellated but split by a rule governing thesize and shape of areas corresponding to different microstructures whichin one embodiment can vary and which in another embodiment of this canbe used to carry an additional hidden code or signature that can be madecharacteristic of the form of origination or the application forexample.

[0098] A further useful (not limiting) embodiment of this device iswhere the sub areas are subdivided by a rule, complex or simple,generating the shape and parameters of the sub areas and also therelative area split of the sub areas in order to provide the desiredrelative brightness of the various diffractive view channels of thedevice.

[0099] A useful embodiment is where the graphical subdivisions betweenchannels are in the form of curved lines, elongated in one direction butof a size in another direction to stay below the limit threshold ofhuman vision (10 to 75 micron) typically such that the length of thefeature will be between 2 and 10 times its width. Usefully a curved lienstructure also optionally with irregular aspects reduces noise from anyperiodic effects in the substructure seen in earlier techniques (such asregular pixel patterns or tracks). Usefully the curved lines or othergeometrical shapes will be of varying size in on direction (e.g. varyinglength).

[0100] Useful embodiments of the above method for subdividing adiffractive structure will be for a diffractive image to produce aswitching effect where the diffractive structures when. subdivided havea much higher diffraction efficiency than the multiple diffractivestructures when superimposed on one another where the diffractiveefficiency is lower.

[0101] A useful device is where the device consists of areas ofdiffractive structure with the same spatial frequency but differentgrating profiles arranged such that the areas have different diffractiveefficiencies in different grating orders (+1 and −1) such the opticaldevice replays an image visualised by an observer by contracts reversalon rotating the device through 180 degrees in its own plane due to thechange in diffraction efficiency between the two non symmetrical gratingstructures.

[0102] Turning now to FIG. 13, there is illustrated an opticalmicrostructure producing under illumination a or more defined graphicalimages generated by a process of diffraction and visible to an observerfrom different angles around the device, characterized such that the subarea is split into discrete sub areas of a size not discernable to anobserver, (typically 10 to 75 micron), each collection of sub areascontaining the diffractive or sub micron microstructure applicable toone viewable diffracted image, the graphical areas of sub-division beingvariable and consisting of dots taken out of a full area, lines orelongated dots or short lines taken out of a full area, curved lineartracks, wavy line patterns.

[0103] A further useful embodiment of a device. Such as that in FIG. 12Ais where the sub areas are sub divided by a rule, complex or simple,generating the shape and parameters of the sub areas and also therelative area split of the sub areas in order to provide the desiredrelative brightness of the various diffractive view channels of thedevice.

[0104] A useful embodiment is where the graphical sub divisions betweenchannels are in the form of curved lines, elongated in one direction butof a size in another direction to stay below the limit threshold ofhuman vision (10 to 75 micron) typically such that the length of thefeature will be between 2 and 10 times its width. This is shown in FIG.14. This formation of sub areas is useful to allow a useful area ofdiffractive structure to gain useful level of efficiency and to reducethe number of boundaries and yet also to keep one dimension of thestructure small enough to prevent degradation of the graphicaldiffracted image by the sub structure. Usefully a curved line structurealso optionally with irregular aspects reduces noise from any periodiceffects in the sub structure seen in earlier techniques (such as regularpixel patterns or tracks). Usefully the curved lines or othergeometrical shapes will be of varying size in one direction (e.g.varying length) in order to reduce summation effects and eliminate upany overall pattern effect that may be visible to an observer.

[0105] Useful embodiments of the above method for sub dividing adiffractive structure will be for an diffractive image to produce aswitching effect where the diffractive structures when sub divided havea much higher diffraction efficiency than the multiple diffractivestructures when superimposed on one another where the diffractiveefficiency is lower.

[0106] A diffractive Optical Device as noted can be arranged toincorporate an optical switching image where the elemental diffractivestructures comprise diffraction gratings modeled to give the diffractivereplay effects of short rainbow hologram slits. These can also containelements that are Fourier holograms producing images focused far fromthe image plane of the device to be read under laser illumination. Suchan arrangement is illustrated in FIGS. 15A and 15B.

[0107] In FIGS. 16A to 16C, there is illustrated a diffractivemicrostructure producing a 3D depth effect (e.g. a 2D/3D effect by theappearance of depth and parallax) or a true three-dimensional effect oran effect similar to stereogram techniques for producing 3D images ofreal subjects. The effect is generated by sub dividing the diffractivestructure into many small sub areas, typically 10 ro 15 or more for a 3Deffect or stereogram effect, an using each small sub areas or assemblyof sub areas to replay a particular view parrallax of the objectdirected into the observers eye. A useful method of area sub divisionsto conceal the small graphical sub divisions is to use a sub divisionrule of different shapes or geometries—here this is a similar techniquein this aspect to the optical devices of FIGS. 12 and 13.

[0108] A useful sub division methodology is where the microstructure issplit into curved lines, as in device 2, or vertically orientated linesand this is shown in FIGS. 17A and 17B. Another useful embodiment iswhere the area of each elemental line (e.g. for the 1^(st) and 2^(nd)leftmost views and say the 14th and 15^(th) right most views of anobject) are adjusted in area to adjust the brightness of thecorresponding diffractive channels to relative to the centre parallaxchannels—for example to give good clarity but sub due depth effects byreducing these channel efficiencies or to give an enhanced depth andparallax effect by enhancing these channel efficiencies.

[0109] A particularly useful aspect of this illustrated embodiment isthe use of the technique to generate an optical diffraction image thatreplays under illumination a three dimensional image in one view andalso replays a second three dimensional image visible on rotating thedevice through 90 degrees in its plane.

[0110] Turning now to FIG. 18, there is illustrated an optical devicemirroring aspects of the devices described above and wherein thediffractive areas of the device have replay characteristics that, whenilluminated and rotated in a plane, an observer sees an apparent motioneffect of movement and/or shape changes. Such effects are useful forsecurity and would incorporate apparent motion of an object, morphing ofan object between various states through a sequence of views,expansions, linear motions, etc. A useful attribute is where theelemental diffractive areas are border and operate together in elementshapes as in the device of FIG. 12 and particularly where these shapesfulfil an additional purpose of authentication.

[0111] Two particularly useful effects are where the elemental area havean apparently random though characteristic pattern generated typicallyas a fractal pattern of areas and where the patterning can containadditional encrypted information adding to the security and to verifythe authenticity of the device. Also we claim a method of verificationwhereby the diffractive device is scanned and the pattern of then areadistribution structures decoded to provide additional characteristicinformation pertaining to the hologram.

[0112] The devices illustrated with regard to FIGS. 19A relate to any ofthe aforementioned devices and in which sub area diffractive structuresare filled by plane diffraction grating structures.

[0113] The device of FIG. 19B arises from the sub area structures of thesaid aforementioned devices being filled with alternative pre-calculatedsub micron diffractive structures. Particularly useful structurescomprise diffractive structures whose replay is limited in parallax inthe one direction and slightly extended in parrallax in anotherdirection (a similar replay to a short element of a rainbow hologram) togive a useful degree of additional view angle in a direction parallel tothe horizontal axis of the device. A useful switching device is made upof an assembly of such structures providing usefully different viewcharacteristics to those of a standard diffraction grating device

[0114] Turning now to FIG. 19C, there is illustrated an optical devicein which one or more components of the elemental diffractive structuresconsist of diffractive devices reconstructing a diffracted image similarto that of a rainbow hologram, whilst other components of the elementalstructures consist of diffraction grating structures.

[0115] A method of incorporating, in the devices of FIGS. 12 and 13, amicroscopic information structure containing graphical information ofsize 250 micron to 2 micron illustrated by FIG. 20. Such informationwould be either occupying a discrete set of zones, for example bywriting into the device of FIG. 12 a set of zones defining one componentdiffractive image, and in the device of FIG. 13 a structure that alsocontains within its fill structure microscopic graphic information. Thisgraphical information would be visible only with magnificationaids—microscope, etc.

[0116] A useful example of this is where the microscopic securitygraphics is arranged in shapes enclosing and modulating a diffractivearea—a useful form of diffractive area would be an area replaying underillumination a rainbow holographic type structure—this has advantages inthat incorporating microscopic information into this type of structureis extremely difficult both using conventional laser interferometrytechniques (as the image will be degraded and destroyed by focus errorsand speckle) and also by using dot matrix or mechanical recombinationtechniques (which are useful for recording areas of plane diffractiongratings). A useful text size would be 10 micron to 200 micron.

[0117] Another useful aspect of this invention is where the micro-textand micro-graphics are very small (size 1. to 10 micron) and areincorporated within the diffractive or holographic sub-microndiffractive structure within the diffractive line structure—one exampleof a suitable diffractive structure would be a diffraction grating butanother a particularly useful example would again be where thediffractive structure corresponds to that of a rainbow hologram, or openaperture hologram or a diffractive element reconstructing a laserviewable out of plane image where the structure would be very difficultto produce with other techniques.

[0118] A useful alternative embodiment that would be incorporated withOptical devices 1 and 2 would be where one of the sub division graphicshapes used for one or several of the sub division areas would be in theshape of micro writing or microtext.

[0119] As with the device of FIG. 20, and further illustrated in FIG.21, where many identical images of micro images or micro graphics arearranged in a regularly spaced two dimensional array within the substructure of the diffractive device—typically each micro-image will beof a size where the details cannot be resolved by the unaided humaneye—typical size range 200 micron to 10 micron. Elemental details onsuch micro images will be down to say 1 micron. A particularly usefulform of micro image is where the micro image is reversed dark out of adiffractive area, typically a diffraction grating or holographic likediffractive area. Another useful embodiment is here the diffractive areais an achromatic replay diffractive element or is diffusingnon-holographic structure. In these examples the micro images would bereversed out of the microstructure

[0120] A preferred and useful method of authenticating such a structureis to use an array of small lenslets, known as micro-lenses whose pitchis matched to the regular array of the micro-images and whose focalpoint is on the plane of the device. In this case when the micro lensarray is overlaid on top of the diffractive device the micro lenses andmicro images cooperate via a moiré type effect to generate a magnifiedimage or images of the micro image—in the case of perfect alignment onesingle enlarged image will be seen whilst in the case of some angularmis-alignment a number of smaller images will be seen whose size,orientation and position is determined by the moire beat effect betweenthe micro image array and lens array as successive areas produce in andout of phase images.

[0121] This device would also be useful if the elemental areas of microimages were created out of areas of diffractive structure such as planediffraction gratings or rainbow or Fourier type diffractive devices. Auseful authentication device is the use of a diffractive lens (a fresnelzone plate) rather than a micro lens structure as shown in FIG. 20B—typically this would have the advantage of being manufacturable byroll embossing.

[0122] Another particularly useful embodiment of the hologram is wherethe basic microstructure is ordered with a secondary pattern on the 50micron to 170 micron scale which appears to be uniform—as in FIGS. 21Ato 21C—such as a pattern of pixels, dots or lines. However, thepositions of individual elements are subtly moved from a regular arrayposition by a small imperceptible amount. This additional patterning isdesigned to be undetectable to the eye and generally undetectable underclose examination. However, the patterning is designed to be decode by amatched film overlay reader containing a matched but regular patternWhen overlaid and viewed in cooperation with the decoder film the twopatterns cooperate to generate moiré fringes‘the displaced elements andthe hidden code therein can then be seen clearly by an observer as aninterruption and displacement within the moire fringes. This patternrelies upon the spatial frequency of the microscopic pattern within thediffractive structure and the spatial; frequency of the dot or linepattern in the decoder being the same or closely matched. Usually thecontrast of the decoded image will be optimised by using a reverse orcontrast enhancing film decoder.

[0123] The method of optical microstructure manufacture detailed above,and with regard to the device examples of FIGS. 12 and 13 can also becan also be expanded to form a non pixel based/non track method of greyscaling as shown in FIG. 22—various of the arbitrarily shapeddiffractive features would contain a non diffractive scatteringstructure designed to scatter incident light to form a print typeeffect. Using methods for forming the devices of FIGS. 12 and 13, thesenon-diffractive scattering structures could form one component of thesub divisions of the optical device. This could be used to provide ascattering permanent image replaying with the diffractive structure orto form areas composed of white scattering features. A grey scale imagecould be produced by modulating the areas of the elemental scatteringfeatures to alter the fill factor by elemental area of scatterer toproduce grey scale tones. Arbitrary micrographics could be incorporatedby altering either the shape of the elemental areas to form graphicaleffects or by incorporating graphical images into the structure submicron diffusing structure.

[0124] The methods as employed for forming any of the devices above,where a rule of graphics arrangement in the artwork on a small scale(500 micron to 1 micron) is arranged in a pattern whose arrangementcarries encoded information, typically arranged according to fractals,and having a so called Digital Watermark encoded therein is illustratedin FIG. 23.

[0125] Finally, with regard to FIG. 24, there is illustrated a devicewherein certain areas of the device the diffractive structures areholographic optical elements generating lens and focusing effects andwhich can be incorporated into any of the above-mentioned devices.Advantageous embodiments would comprise areas of multiple lenses allforming focused images at particular direction and distance from thedevice and in a particularly advantageous embodiment this can form anencoded message.

[0126]FIGS. 25 and 26 illustrate, by means of flow diagrams, processsteps according to methods embodying the present invention and, in turn,would correspond to functional block diagrams of related apparatus.

[0127] It should be appreciated that the invention is not restricted tothe details of any of the foregoing embodiments.

1. A method of producing an optical device by means of electron beamlithography and including the step of varying the characteristics of theelectron beam spot during formation of the device.
 2. A method asclaimed in claim 1 including the step of varying the size of theelectron beam spot.
 3. A method as claimed in claim 1 and including thestep of varying the shape of the electron beam spot.
 4. A method asclaimed in claim 1, and including the step of forming the electron beamspot in a substantially rectangular form.
 5. A method as claimed inclaim 4 and including the step of selectively varying either one, orboth, of the lateral dimensions of the said rectangular form.
 6. Amethod as claimed claimed claims 1 and including the step of rotatingthe electron beam spot.
 7. A method of producing a diffractive opticaldevice and which includes the exposure of arbitrary binary structuresspecified by bitmaps.
 8. A method as claimed in claim 7 and includingthe step of analyzing a microscopic exposure pattern for the diffractivedevice.
 9. A method as claimed in claim 7 including the step of fillinga graphic area of the device with a sub-micrometer structure and in aform pre-determined by a data file retrieved from data library storagemeans.
 10. A method as claimed in claim 9 and including the step ofcombining a graphics file defining an image to be found, a descriptor ofeach graphical area, and a data library containing the predeterminedsub-micrometer structures.
 11. (Cancelled)
 12. A method as claimed inclaim 11 and including the steps of determining colored areas in a imagespecified by means of a black and white bitmap and dividing the saidareas into elementary sub-areas corresponding to electron beam exposurespots.
 13. A method as claimed in claim 12 and including the step ofincorporating the said black and white bitmap in a manner wherein thewhite areas are arranged to represent an exposure area.
 14. A method asclaimed in claim 12, and including the step of defining input parameterson the basis of minimum or maximum electron beam spot size and themaximum ratio between different dimensions of the electron beam spot.15. A method as claimed in any one of claims 12, and including the stepof defining a permitted exposure mode adjacent a border of an exposedarea on the basis of the relationship between the electron beam exposurespot and the border area.
 16. A method as claimed in claim 15, andincluding the step of controlling the said relationship such that theelectron beam exposure spots do not exceed the boarder by more than 0.5of the minimum electron beam spot area.
 17. A method as claimed in claim12, and including the step of defining a filling level of exposureadjacent the said border.
 18. A method as claimed in claim 13, andincluding the step of determining an optimized filling of the exposurearea when-the electron beam exposure spot is arranged to provide anexposure time determined by external data.
 19. A method as claimed inclaim 12, and including an intermediate step of running a set of datafiles in an exposure simulation software program serving to check forrun time and run integrity.
 20. A method as claimed in claim 19, andincluding the step of employing the said data files to write anarbitrary diffractive microstructure by employing the said data files asan input exposure control to an electron beam lithograph.
 21. A methodas claimed in claim 7 and including the step of assembling inputgraphics as one of a plurality of possible digital graphical fileformats, defining a specific area filling as at least one of the colorsin the image for the generation of specific predetermined elementaryoptical microstructure.
 22. A method as claimed in claim 21 andincluding the step of preventing the structure from exceeding a definingborder and on the basis of tolerance defined by minimum electron beamsspot size.
 23. A method as claimed in claim 21, and including anintermediate step of running a set of data files in an exposuresimulation software program in order to determine run time and runintegrity.
 24. A method as claimed in claim 21, and including the stepof writing an arbitrary resolution area of a set of opticalmicrostructures defined by the said data library and by means of thesaid data files serving as input exposure control to an electron beamlithograph.
 25. A method as claimed in claim 1 and including the step ofholographic embossing so as to form a surface relief structure. 26-28.(Cancelled)
 29. An apparatus for producing diffractive optical devicesand/or holographic devices by means of electron beam lithography andincluding an electron beam lithograph, controlling and processing means,means for varying the characteristics of the electron beam spot duringformation of the device, and wherein the processing means is arrangedfor compiling and pre-processing data and for providing optimization andallocation control.
 30. An apparatus as claimed in claim 29 includingmeans for varying the size of the electron beam spot.
 31. An apparatusas claimed in claim 29 and including means for varying the shape of theelectron beam spot.
 32. An apparatus as claimed in claim 29, andincluding means for forming the electron beam spot in a substantiallyrectangular form.
 33. An apparatus as claimed in claim 32 and includingmeans for selectively varying either one, or both, of the lateraldimensions of the substantially rectangular spot.
 34. An apparatus forproducing a diffractive optical device and including means for providingexposure of arbitrary binary structures specified by bitmaps.
 35. Anapparatus as claimed in claim 34 and including means for analyzing amicroscopic exposure pattern for a diffractive device.
 36. An apparatusas claimed in claim 34 including means for filling graphic area ofdevice with a sub-micrometer structure as a form pre-determined by adata file with a data library storage means.
 37. An apparatus as claimedin claim 36 and including means serving to combine a graphics filedefining an image to be found, a descriptor of each graphical area, anda data library containing the said various predetermined sub-micrometerstructures.
 38. (Cancelled)
 39. An apparatus as claimed in claim 34 andincluding means for determining colored areas in an image specified bymeans of a black and white bitmap and means for dividing the said areasinto elementary sub-areas corresponding to electron beam exposure spots.40. An apparatus as claimed in claim 39 and including means forincorporating the said black and white bitmap in a manner wherein thewhite areas are arranged to represent an exposure area.
 41. An apparatusas claimed in claim 39, and including means for defining inputparameters on the basis of minimum or maximum electron beam spot sizeand the maximum ratio between different dimensions of the electron beamspot.
 42. An apparatus as claimed in claim 39, and including means fordefining a permitted exposure mode adjacent a border of an exposed areaon the basis of the relationship between the electron beam exposure spotand the border.
 43. An apparatus as claimed in claim 42, and includingmeans for controlling the said relationship such that the electron beamexposure spots do not exceed the border by more than 0.5 of the minimumelectron beam spot area.
 44. An apparatus as claimed in claim 39, andincluding means for defining a filling level of exposure adjacent thesaid border.
 45. An apparatus as claimed in claim 39 44, and includingmeans for determining an optimized filing of the exposure area when theelectron beam exposure spot is arranged to provide an exposure timedetermined by external data.
 46. An apparatus as claimed in claim 39,and including means for running a set of data files in an exposuresimulation software program serving to check for run time and runintegrity.
 47. An apparatus as claimed in claim 46, and including meansarranged for employing the said data files to write an arbitrarydiffractive microstructure by employing the said data files as an inputexposure control to an electron beam lithograph.
 48. An apparatus asclaimed in claim 47 and including means serving to assemble inputgraphics as one of a plurality of possible digital graphical fileformats and defining a specific area filing as colors in the image forthe generation of specific predetermined elementary opticalmicrostructures.
 49. An apparatus as claimed in claim 48 and includingmeans for determining that the structure does not exceed a definingborder and on the basis of tolerance defined by minimum electron beamsspot size.
 50. An apparatus as claimed in claim 48, and including meansarranged for running a set of data files in an exposure simulationsoftware program in order to determine run time and run integrity. 51.An apparatus as claimed in claim 48 and including means arranged forwriting an arbitrary resolution area of a set of optical microstructuresdefined by the said data library and by means of the said data filesserving as input exposure control to an electron beam lithograph. 52-55.(Cancelled)
 56. An optical diffractive device for producing graphicalimages and including a microstructure corresponding to each graphicalimage and located in small discrete areas within the device.
 57. Adiffractive device having a surface relief structure which whenilluminated by a white light source generates by a process of opticaldiffraction two or more defined graphical images visible to an observerform different observation directions around the device which producethe visual effect of a defined image switch between two or more visuallydistinct diffractive images wherein the microstructure corresponding toeach graphical image is defined in an array of discrete small areascharacterized such that the graphical areas containing themicrostructure are of a flexible graphical shape.
 58. A device asclaimed in claim 56 and arranged such that the size of themicrostructures are of a size smaller than normal resolution of thehuman eye.
 59. A device as claimed in claim 57 and including subdividedareas corresponding to respective viewing channels and which are dividedbetween defined graphical areas corresponding to different diffractedviews employing graphical shapes of different sizes and shapes.
 60. Adevice as claimed in claim 59 and exhibiting a pseudo random patternhaving shape and area governed by fractal geometry and wherein the areasoffer a discernable statistical profile.
 61. An optical diffractivedevice having a sub-area divided into further discrete sub-areas each ofwhich is not discernable to the human eye, and wherein each collectionof discrete sub-areas is arranged to contain the diffractivemicrostructure associated with a viewable diffracted image.
 62. Anoptical microstructure arranged to produce under illumination two ormore defined graphical images generated by a process of diffraction andvisible to an observer from different angles around the device,characterized such that the sub area is split into discrete sub areas ofsize not discernable to an observer, each collection of sub areascontaining the diffractive or sub micron microstructure applicable toone viewable diffracted image, the graphical areas of sub-division beingvariable and consisting of dots taken out of a full area, lines orelongated dots or short lines taken out of a full area, curved lineartracks, wavy lien patterns.
 63. A device as claimed in claim 62 whereingraphical sub-divisions between viewing channels of the device are inthe form of elongated features such as curved lines and having adimension in another direction determined to remain below the thresholdlimit allowing for human vision.
 64. A device as in claim 63 wherein thedevice consists of areas of diffractive structure with the same spatialfrequency but different grating profiles arranged such that the areashave different diffractive efficiencies in different grating orders (+1and −1) such the optical device replays an image visualized by anobserver by contrast reversal on rotating the device through 180 degreesin its own plane.
 65. A device as claimed in claim 63, wherein thelength of the feature is in the range of 2-10 times its width.
 66. Adevice as claimed in claim 63, wherein the features are of a differentsize in at least one dimension.
 67. A device as claimed in claim 62 andwherein the structure comprises a diffraction grating arranged toprovide a diffractive replay effect of a short rainbow hologram slit.68. A device as claimed in claim 67 and wherein the structure includeselements comprising Fourier holograms arranged to produce images focusedfar from the image plane of the device.
 69. An optical diffractivedevice including a diffractive microstructure divided into a pluralityof small sub-areas and wherein each small sub-area, or group of suchsub-areas, is arranged to replay a view in parallax of the imagedirected to an observer.
 70. An optical microstructure producing underillumination two or more defined graphical images generated by a processof diffraction and visible to a observer from different angles andarranged for producing a 3D depth effect by subdividing the diffractivestructure into many small sub areas and using each small sub area orassembly of sub areas to replay a particular view parallax of the objectdirected into the observer's eye.
 71. (Cancelled)
 72. A device asclaimed in claim 70 wherein diffractive areas of the device are arrangedwith replay characteristics that, when illuminated and rotated, providefor an apparent motion effect and/or shape of variations.
 73. A deviceas claimed in claim 72 and including an elemental area offering apattern generated as a fractal pattern of areas including additionalencrypted information.
 74. A device as claimed in claim 73 and includingsub-area diffractive structures each completed with plane diffractiongrating structures.
 75. A device as claimed in claim 73 and includingsub-area structures completed with pre-determined sub-micron diffractivestructures.
 76. A device as claimed in claim 75 and includingdiffractive structures whose replay is limited in parallax in the onedirection and slightly extended in parallax in another direction.
 77. Adevice as claimed in claim 70, wherein the said sub-micro diffractivestructures comprise structures arranged to offer replay characteristicslimited in parallax in one direction and slightly extended in parallaxin another direction.
 78. An optical diffractive device including one ormore components arranged for reconstructing a diffracted image, whilstothers of said component comprise diffraction grating structures.
 79. Adevice as claimed in claim 78 and including a microscopic informationstructure containing graphical information.
 80. A device as claimed inclaim 79 wherein the said graphical information is arranged to occupy adiscrete set of zones and which graphical information is arranged to bevisible only through magnification.
 81. A device as claimed in claim 80and including microscopic security graphics arranged for enclosing andmodulating a diffractive area.
 82. A device as claimed in claim 81,wherein the said diffractive area comprises an area arranged to reply arainbow holographic type structure.
 83. A device as claimed in claim 80,wherein text and graphics are incorporated within the diffractivestructure.
 84. A optical diffractive device as claimed in claim 81wherein at least one subdivided graphic shape employed for at least oneof a plurality of subdivided areas comprises micro-writing ormicro-text.
 85. An optical diffractive device as claimed in claim 79,and wherein a plurality of identical images comprising micro-images ormicro-graphics are arranged in a regularly spaced two-dimension arraywith a substructure of the diffractive device.
 86. A device as claimedin claim 85, wherein the micro-image comprises a micro-image reversedout of a diffractive area.
 87. A device as claimed in claim 85, whereinthe diffractive area comprises an achromatic replay diffractive elementor a diffusing non-holographic structure.
 88. An optical microstructurearranged to produce under illumination two or more defined graphicalimages generated by a process of diffraction and visible to an observerfrom different angles around the device and including a microstructureordered with a secondary pattern and wherein the positions of theindividual elements are offset with regard to a regular array positionby an amount undetectable to the human eye.
 89. A device as claimed inclaim 88 wherein a plurality of arbitrarily shaped diffractive featurescontain a non-diffractive scattering structure arranged to scatterincident light to form a print-effect.
 90. A device as claimed in claim89, wherein the said non-diffractive scattering structures form onecomponent of the sub divisions of the device.
 91. A device as claimed inclaim 88, and including a rule of graphics arrangement of the graphicalimage on a scale of 1 micron-500 microns and arranged in a patternserving to include encoded information.
 92. A device as in claim 91wherein the graphical arrangement is in the form of lines of variablespacing or shape.
 93. A device as in claim 91 wherein the graphicalarrangement is in the form of dots of variable size or position.
 94. Adevice as claimed in claim 91, wherein the encoded information isarranged according to fractals and with a digital watermark encodedtherein.
 95. A device as claimed in claim 88, wherein at least some ofthe diffractive structures comprise holographic optical elementsgenerating lens and focusing effects.
 96. A method of verifying anoptical diffractive security device and including the step of scanningthe said device and wherein the pattern of area distribution structuresdetected is decoded to provide additional characteristic securityinformation pertaining to the device.
 97. A method of verifying anoptical diffractive security device as in claim 96 including the step ofoverlaying the said device with a semi transparent overlay containing agraphical pattern of lines or dots arranged by a rule of graphicalarrangement similar to that used in the diffractive device, wherein thepattern of area distribution of structures is revealed and decoded by amoire interference effect to visualize a covert security image containedwithin the graphical arrangement of structures with the diffractivedevice.
 98. A diffractive device as in claim 96 further characterizedthat the covert graphical arrangement is contained within thearrangement of the areas of the diffractive structure.
 99. A diffractivedevice as in claim 96 further characterized that the covert graphicalarrangement is contained within the arrangement of the areas of ademetallisation pattern.
 100. A method of verifying an opticaldiffractive security device and employing an array of micro-lenseshaving a pitch matched to an array of micro-images within the device andsuch that its focal point is on the plane of the device, and includingthe step of overlaying the micro lens array on top of the diffractivedevice such that the micro-lenses and micro-images generate a magnifiedimage of the micro-image.
 101. An authentication device for use with adiffractive optical security device and having a micro-lens structurearranged to overlay micro-images of the diffractive device.
 102. Anauthentication device for use with a diffractive optical security deviceand having a diffractive lens structure arranged to overlay micro-imagesof the diffractive device. 103-104. (Cancelled)